This application claims priority to German application 100 30 260.2, filed Jun. 20, 2000, herein incorporated by reference.
1. Field of the Invention
The invention relates to the combined use of reactive amines in combination with metal salts for the preparation of polyurethane foams (PUR foams).
2. Background of the Invention
Owing to their outstanding mechanical and physical properties, polyurethane foams are used in a very wide range of areas. A particularly important market for various types of PUR foams, such as conventional ether- and esterpolyol-based flexible foams, cold foams (frequently also referred to as HR foams) and rigid foams, and foams whose properties are between these classifications, such as, for example, semirigid systems, is the automotive industry. For example, rigid foams are used as roof linings, ester foams for interior cladding of the doors and for punched-out sun visors, and cold and flexible foams for seat systems.
In recent years, the requirements set by the automotive manufacturers for their foam suppliers have become substantially more stringent, especially with regard to an emission specification. Whereas in the past attention was focused only on the fogging behavior of the foams (DIN 75 201, determination of the fogging behavior of materials for interior automotive trim), today the content of volatile organic compounds (VOC) is also a subject of analytical determinations (Volkswagen central standard 55 031, Daimler Chrysler PB VWT 709). The Daimler-Chrysler method requires the assignment of the emissions to individual chemical compounds in addition to the quantitative determination of the VOC and fog value.
The prior art involves the use of tin octanoate in the preparation of flexible PUR foams based on polyetherols (George Woods, The ICI Polyurethanes Book, Wiley Publishers, page 45, and Ron Herrington, Flexible Polyurethane Foams, Dow Chemical, page 2.30). The tin octanoate serves as a catalyst in the reaction of isocyanates with polyols (also referred to as a gel catalyst) via a complex transition state. During the preparation of the foam, the tin octanoate hydrolyzes and liberates the 2-ethylhexanoic acid. This decomposition is desired because the back-reaction of the urethane bond to the starting materials is suppressed. The ethyl branching of the octanoate is of decisive importance for the formation of the desired ligand complex.
The use of zinc stearate as an internal lubricant in the preparation of RIM foams (reaction injection molding) is widely mentioned in the patent literature. The patents U.S. Pat. Nos. 5,008,033; 5,212,209; EP-A 0 490 342 and WO 96/22182 may be mentioned by way of example. The use of metal salts of higher carboxylic acids, preferably zinc stearate, and their additions for compatibilization in the RIM mixture are claimed in these publications.
It should be pointed out here that polyurethane RIM systems have substantial differences compared with the foam systems according to the invention. RIM systems are compact moldings or microcellular systems but by no means open-pore foams. Accordingly, the densities of the two systems differ dramatically. RIM moldings have densities of  greater than 700 kg/m3 and the PUR foams according to the invention have densities of  less than 100 kg/m3, in particular  less than 50 kg/m3. The catalysis of RIM systems is substantially different. Instead of tin octanoate, dibutyltin laurate is typically used in RIM systems. As shown by the comparative examples, neither zinc stearate (zinc salt of octadecanoic acid) nor zinc oleate (zinc salt of 9-octadecenoic acid) or zinc 12-hydroxystearate has a substantial advantage.
A conventional flexible foam having the density 25 kg/m3 typically has the following VOC emissions: total value 800 ppm, classified as 550 ppm of BHT (bis-2,6-tert-butyl-4-hydroxytoluene), 200 ppm of 2-ethylhexanoic acid, 20 ppm of tertiary amines, 10 ppm of siloxanes and 20 ppm of unspecified compounds. Of course, the emissions are highly dependent on the respective formulation but BHT and 2-ethylhexanoic acid are always the main components. BHT typically originates from the polyol and isocyanate. The manufacturers of these raw materials have recently been offering BHT-free grades of their products. Using these raw materials, foams having a VOC value of about 250 ppm can be prepared.
Since automotive manufacturers have now specified a VOC guide value of 100 ppm, which is to be reached in the next few years, there is an urgent technical necessity for reducing the 2-ethylhexanoic acid emission and the amine emission.
2-Ethylhexanoic acid is a decomposition product of tin octanoate, which usually acts as a catalyst of the polyurethane reaction.
The amine emission from foams constitutes the second problem which at present cannot be satisfactorily solved. There are several possible approaches for reducing the amine emission. Thus, high molecular weight amines can be used. Owing to the high MW, these compounds are no longer volatile and are not emitted from the foam. At the same time, however, the mobility of the molecules is limited for the same reason, so that the catalytic effect is substantially reduced. Furthermore, the slower curing of the skin constitutes a further problem. The other alternatives are reactive amines which are provided with OH or NH functional reactive structures and are thus incorporated into the PU matrix by reaction. Here too, emission is prevented by the chemical bonding to the polymer. A disadvantage of the compounds available on the market is recatalysis. By lowering the activation barrier, catalysts accelerate not only the forward reaction but also the reverse reaction. Foams which were prepared using reactive amines thus show substantially poorer aging behaviorxe2x80x94in particular in the humid aging test.
The prior art describes no possibility for preventing this recatalysis.
An object of the present invention to overcome the abovementioned problems.
In a first embodiment of the invention, the abovementioned problems are solved by using reactive amines of the general formula (I) 
where
n is 1 to 4,
R1 and R2 are
xe2x80x94(CH2xe2x80x94CH2xe2x80x94O)xH,
xe2x80x94(CH2xe2x80x94CH(CH3)xe2x80x94O)xH, or
xe2x80x94((CH2xe2x80x94CH(CH2xe2x80x94CH3)xe2x80x94O)xH, and
x is 0, 1, 2, 3 or 4,
with the proviso that,
at least one of the indices x in the molecule is greater than 0,
in combination with metal salts or their solutions in aqueous or organic solvents for the preparation of polyurethane foams.
A preferred embodiment of the present invention comprises the use of a specific metal salt catalyst in combination with the reactive amine. The metal salt catalyst has a greater tendency to catalyze the so-called gel reaction (urethane formation) in the preparation process for PU foam. The amine catalyst, optionally in combination with further amines, is on the other hand more responsible for accelerating the blowing reaction (finally urea formation). Also in systems of the prior art, a mixture of the two catalysts is used which leads to a useful balance between the two partial reactions. It is possible to reduce the amine emission, for example, not only by the use of less amine catalyst, since the two partial reactions run out of equilibrium as a result. This is also true for the use of metal salt catalysts for the gel reaction.
A further substantial advantage of the present invention is the surprisingly substantially altered balance between the catalysis of the two partial reactions. Owing to the increased acceleration of the driving reaction, the amount of amine catalyst to be used can be substantially reduced compared with the prior art. With the use of the specific reactive amine having two reactive centers, the tendency to emission is completely suppressed. Particularly surprising is the effect that, as a result of the combination of said catalysts, no-deterioration with regard to humid aging was observed. The foams prepared using this combination do not differ with regard to their aging properties from present-day foams produced using tin octanoate and volatile amines.
In the context of the present invention, the reactive amines, which can be used in the preparation of polyurethane foam, are particularly preferably selected so that n in the general formula (I) is 3. In the context of the present invention, it is also particularly preferred if x in the general formula (I) is 1 and y is likewise 1.
In the context of the present invention, the reactive amine used is particularly preferably an N,N-dimethy-Nxe2x80x2,Nxe2x80x2-2-hydroxy(propyl)-1,3-propylenediamine.
The reaction 
takes place mainly on the sterically less hindered side of the alkylene oxide (to an extent of about 90%). There is also always a reaction product which is formed by attack at xe2x80x9c2xe2x80x9d (to an extent of about 10%). 
By adding the metal salts in particular of 2-ethylhexanoic acid and/or of ricinoleic acid, it is possible to prepare foam having lower emission values, better flameproof properties and less odor. In addition, the curing of the foam surface is accelerated. These advantages can be achieved without changing the other physical properties, such as, for example, density, hardness, resilience or compressive strength.
Octanoates and/or ricinoleates of the metals of the 1st, 2nd or 4th main group and of the 1st, 2nd or 8th subgroup of the Periodic Table are particularly suitable. Particularly preferred in the context of the present invention are zinc and/or tin, so that tin-free foams can also be prepared.
Among the cations, tin is particularly preferred, especially in the divalent form, since tin ricinoleate is present in liquid form at room temperature.
The likewise preferred zinc salt of 2-ethylhexanoic acid and/or ricinoleic acid can be predissolved in the activator solution, consisting of water, tertiary amine, silicone stabilizer and optionally emulsifier. The direct metering of the solid octanoate and/or ricinoleate into the foaming component leads to a foam having an irregular cell structure. Since many expanders have only direct metering, a product in which the zinc salt of 2-ethylhexanoic acid and/or ricinoleic acid is present in dissolved form, or the tin salt in liquid form, constitutes a considerable improvement. Anhydrous solutions are preferable because otherwise the water from the solvent reacts with the isocyanates and therefore has to be included in the calculations for the formulation. Moreover, some transition metal salts have only limited stability to hydrolysis.
In principle, the combinations of from about 5 to about 50% by weight, based on the mass of solvents and salt, of an ethoxylated fatty alcohol having a straight and branched alkyl chain and between 10 and 18 carbon atoms and less than 30 ethylene oxide units with from about 5 to about 30% by weight of a tertiary amine are suitable as anhydrous solvents for the zinc salt, in particular of 2-ethylhexanoic acid and/or of ricinoleic acid. In solvents characterized in this manner, up to about 60% by weight of the metal salt can be dissolved to give a clear solution.
A combination of from 5 to 32% by weight of a fatty alcohol having a straight and branched alkyl chain and between 10 and 18 carbon atoms and less than 20 ethylene oxide units with from 5 to 30% by weight of the reactive amine is preferred.